Disarray in the brain

Our brain never rests completely: Even in situations without noticeable external input thousands of neurons are firing in different parts of the brain, a phenomenon neuroscientists refer to as “spontaneous activity” or even neural noise.

Scientists from the University of Lübeck now have discovered that the structure of this seemingly random noise in fact conveys double meaning: On the one hand spontaneous activity becomes more random with increasing age. On the other hand the current degree of irregularity affects the perception of our auditory environment.

Our brain changes as we get older: Connections between brain areas are rebuild, white matter is getting reduced, and local connections strengthened. These processes of reconstruction also affect spontaneous activity: “Neural noise becomes more random with increasing age”, explains Leonhard Waschke who conducted the study. “In this case it is not of importance if the brain is seemingly at rest or if it is busy solving a challenging cognitive task. The growing irregularity of neural noise thus seems to be a very stable feature of aging brains.”

At the same time, neural noise is in no way constant across time. The structure of spontaneous activity varies from second to second. Hence, moments of relative regularity at times are followed by moments of relatively large irregularity.

To investigate how the current degree of irregularity affects human perceptions and decisions, the scientists around Prof. Dr. Jonas Obleser chose an approach that may seem a little odd at first glance. While they recorded the brain activity of volunteers with the help of the electroencephalogram (EEG), these volunteers compared short tones regarding their pitch.

One thing however, was special about those tones: They were identical. Prof. Obleser explains: “Although human perception is very precise. It is not comparable to a machine which for the same input will always produce the very same output. Our perception is a highly subjective process that is affected by a number of cognitive and neuronal mechanisms, and hence also by the structure of neural noise.”

As a matter of fact, all volunteers perceived pronounced differences between the two identical tones and chose one of both as higher in pitch at the end of each trial. This decision was substantially influenced by the structure of spontaneous activity. The more irregular neural noise was during times of tone presentation, the more likely volunteers were to perceive the respective tone as higher in pitch.

The neuroscientists from the “Auditory Cognition” group describe the underlying mechanism like this: “If the auditory cortex is in a state of high irregularity, a new stimulus is processed more thoroughly and results in a clearer representation in the brain.” This seems to be enough to shift the perception in favor of the respective tone.

In a current study the scientists from Lübeck investigate this process in more detail by selectively presenting auditory material when the auditory cortex either is in an especially regular or irregular state.

https://www.nature.com/articles/s41598-017-17766-4